Present and future wireless communication systems include LTE, LTE-A and LTE-A-beyond systems. Measurements and Channel State Information (CSI) feedback are one focus area in LTE-A studies. Measurements and CSI feedback are typically based on various reference signals (RSs). In downlink transmission of LTE-A system, there are reference signals for UE to perform channel/signal estimation/measurements for demodulation of Physical downlink control channel (PDCCH) and other common channels, as well as for some measurements and feedbacks. The reference signals include the Common/Cell-specific Reference Signal (CRS) inherited from the Rel-8/9 specification of E-UTRA. A dedicated/de-modulation reference signal (DMRS) can be transmitted together with the Physical downlink shared channel (PDSCH) in Rel-10 of E-UTRA. The DMRS is used for channel estimation during PDSCH demodulation.
In Rel-10, Channel Status Indication Reference Signal (CSI-RS) is introduced in addition to CRS and DMRS. CSI-RS is used for Rel-10 UEs to measure the channel status, e.g., for multiple antennas cases. PMI/CQI/RI and other feedbacks may be based on the measurement of CSI-RS for Rel-10 and beyond UE. The PMI is the precoding Matrix indicator, and the CQI is channel quantity indicator, and the RI is rank indicator of the precoding matrix. The CSI-RS in Rel-10 can support up to 8 transmission antennas while the CRS can only support up to 4 transmission antennas in Rel-8/9. The number of CSI-RS antenna ports can be 1, 2, 4, or 8. In addition, to support the same number of antenna ports, the CSI-RS has less overhead due to its low density in time and frequency.